Method and apparatus for molding parts

ABSTRACT

A method and apparatus for injection molding plastic parts is described. In one embodiment, at least two materials are simultaneously injected into a mold. The resulting molded part can include at least two different regions. Each region can have distinct physical properties. Positions of the regions within the molded part can be at least partially controlled by controlling flow fronts of the at least two materials within the mold.

FIELD OF THE DESCRIBED EMBODIMENTS

The described embodiments relate generally injection molding and moreparticularly to injection molding plastic parts with non-homogenousmaterial properties.

BACKGROUND

Injection molding, and injection molding plastic parts in particular,has become a very cost effective method for quickly manufacturing parts.Typically, a single plastic resin is selected and prepared for injectioninto the mold. The result is a plastic part with uniform physicalproperties.

Sometimes a part may be formed from two different plastic resins. Inthis scenario, a first material is injected, then after a predeterminedtime period (usually determined by the amount of time needed for themelted resin to set), a second material is injected. A common term ofart used to describe this method is the “two shot” method. In thismethod, the first material is shot into the mold. The mold can berepositioned and then can receive the second material. The two shotmethod can have associated disadvantages. First of all, there is anincreased cycle time to produce molded part. That is, in order for acomplete part to be produced, it takes one cycle of for the firstmaterial, and then one cycle for the second material. Secondly, there isoften a perceptible line at the interface of the two differentmaterials. Oftentimes, this visible line may be undesirable.

There can be some designs when it is desirous for a single part to havevarying physical properties. For example, a designer may want a firstsection or region of a part to have a first amount of stiffness, and asecond section of the part to have a second amount of stiffness. Singleshot injection molding typically only provides uniform physicalproperties throughout the entire part. On the other hand, a two shotprocess can introduce a second material (providing a second physicalproperty) but with added cycle time, and therefore, added cost. Further,the resulting product may no longer be visually acceptable due tovisible interface lines between the first and second materials.

Therefore, what is desired is a cost effective way to produce aninjection molded part with non-uniform material characteristics in ashort process.

SUMMARY OF THE DESCRIBED EMBODIMENTS

This paper describes various embodiments that relate to an injectionmolding method and apparatus. The methods and apparatus disclosed canproduce plastic parts with non-uniform physical properties whileappearing smooth and uniform.

In one embodiment, a method for molding a part includes selecting afirst and a second material, simultaneously injecting the first andsecond materials and controlling the flow fronts of the first and secondmaterials within the mold. In another embodiment, the molded part caninclude a first region having a first physical property and a secondregion having a second material property.

In one embodiment, an apparatus for injection molding a part includes afirst hopper to hold a first material, a second hopper to hold a secondmaterial, a mold including a first gate and a second gate, a firstbarrel coupled to the first hopper and positioned between the firsthopper and the first gate, a second barrel coupled to the second hopperand positioned between the second hopper and the second gate and aninjector controller, coupled to both barrels and configured tocoordinate the injection of materials from the first and second barrelsinto the mold.

Other aspects and advantages of the invention will become apparent fromthe following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1 shows one embodiment of an injection molding system in accordancewith the specification;

FIGS. 2A and 2B show a simplified view of a portion of an injectionmolding system, in accordance with the specification;

FIG. 3 shows another embodiment of an injection molding system inaccordance with the specification;

FIG. 4 shows an exemplary injection molded part 400 made in accordancewith the specification; and,

FIG. 5 is a flowchart of method steps for molding a part withnon-uniform physical properties in accordance with the specification.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

Representative applications of methods and apparatus according to thepresent application are described in this section. These examples arebeing provided solely to add context and aid in the understanding of thedescribed embodiments. It will thus be apparent to one skilled in theart that the described embodiments may be practiced without some or allof these specific details. In other instances, well known process stepshave not been described in detail in order to avoid unnecessarilyobscuring the described embodiments. Other applications are possible,such that the following examples should not be taken as limiting.

The various aspects, embodiments, implementations or features of thedescribed embodiments can be used separately or in any combination.

In the following detailed description, references are made to theaccompanying drawings, which form a part of the description and in whichare shown, by way of illustration, specific embodiments in accordancewith the described embodiments. Although these embodiments are describedin sufficient detail to enable one skilled in the art to practice thedescribed embodiments, it is understood that these examples are notlimiting; such that other embodiments may be used, and changes may bemade without departing from the spirit and scope of the describedembodiments.

A method and apparatus is described for injection molding. The processcan produce a molded part using at least two materials. In oneembodiment, the materials can be plastic resins. The two materials canalso have different physical properties from each other, especiallyafter the materials have been used for making injection molded parts.For example a first material, although similar to second, can produce astiffer molded part through the addition of an effective amount of glassfiller.

An apparatus is provided that enables the simultaneous injection of afirst and a second material into a mold. The first and second materialscan have associated flow fronts within the injection mold. Throughcontrolling the flow fronts, the position of the first and secondmaterials can at least be partially controlled within the mold.Controlling the position of the materials can allow a designer tocontrol at least partially the position of the different material, andhence the position of different physical properties in the final moldedproduct. In other words, controlling the position of the materialswithin the final product can enable the designer to create a productwith area specific physical properties.

FIG. 1 shows one embodiment of an injection molding system 100 inaccordance with the specification. Injection mold 101 can include twopieces. The mold 101 pieces are commonly referred to as a cavity 104 anda core 102. Other embodiments may include more than two mold pieces.Typically, the cavity 104 and core 102 are brought together beforematerial for molding is heated and injected. The cavity 104 and core 102can later separate, after the molding material cools, enabling thefinished part to be ejected from the mold. Hopper 110 can hold a firstmaterial and hopper 112 can hold a second material. In one embodiment,first and second materials can be gravity fed from hoppers 110 and 112into appropriate barrels. Barrels can be used to process the injectionmolding material by mixing, heating and finally injecting material intothe mold 101. Returning to FIG. 1, a first barrel 120 can mix, heat andinject material from hopper 110 into the injection mold while a secondbarrel 122 can mix, heat and inject the material from hopper 112 intothe mold 101.

In one embodiment, the first and second materials can be injectedrelatively simultaneously. That is, both barrels can heat, mix andinject both materials at substantially the same time. In one embodiment,an injection controller 150 can be coupled to the first barrel 120 andthe second barrel 122. The injection controller 150 can be used tocoordinate the injection of the materials from the first 120 and second122 barrels. For example, the injection controller 150 can cause thefirst barrel 120 and the second barrel 122 to simultaneously injectmolding material into the mold 101. Simultaneously injecting twomaterials advantageously produces a single, homogenous part withcomprised of two different materials. In one embodiment, each materialcan possess unique physical properties. In one embodiment, there is novisual difference between the first material and the second material inthe injection molded part. In other words, although two differentmaterials are used, the resulting part can have no visible delineationbetween the two materials (no break lines), the final part appearing asa smooth, continuous part. This is achievable, for example, when theboth the first and second materials are plastic resins, but one of thetwo materials can have an additional filler. The introduction of thefiller can affect the physical properties of the molded part,particularly in the region in the molded part including the fillermaterial.

Injection molded material is typically introduced through “gates” in aninjection mold. FIG. 2A shows a simplified view of a portion of aninjection molding system 200, in accordance with the specification. Inthis illustration, the cavity/core mold 101 from FIG. 1 unit is shown ina simplified manner as mold 210. FIG. 2A is further simplified byshowing only two gates 230 and 232. Other embodiments and systems caninclude more than two gates. A first barrel 220 is coupled to the firstgate 230 and a second barrel is coupled to the second gate 232. Thefirst barrel 220 can convey the first material and the second barrel 222can convey the second material to the mold 210. In other words, materialis injected from barrels 220 and 222 through respective gates 230 and232 into the mold 210.

Positioning of the gates 230 and 232 with respect to the mold 210 canaid in controlling the position of the first and second material withinthe mold. As material is injected through the gates, the material movesthrough the mold by pushing forward “flow fronts”. Flow front 240illustrates the position of material from gate 230 and flow front 242illustrates the position of material from gate 232. As shown in FIG. 2a, the flow fronts are relatively close to the gates. This condition(position of flow fronts 240 and 242) can occur when the first andsecond materials are initially injected into the mold 210.

FIG. 2B shows the progress of the flow fronts 240, 242 in the mold 210.As more material is injected, the flow fronts 240, 242 progress furtherinto the mold 210. Eventually, the flow fronts 240, 242 from the gates230 and 232 will contact each other and form an interface region 250.

When the first material and the second material are similar resins,there may be no visible delineation at or near the interface region 250.In one embodiment, the first material can be a plastic resin with aglass filler and the second material can be a substantially similarpolymer resin to the first material, but without a glass filler. Amolded part that can be produced with these first and second materialsand molding system 200 can appear as a smooth and uniform part, but canbe comprised by two different materials, each material having physicalproperties. The first material can have at least one physical propertydifferent from the second material.

A plastic part can have physical properties, typically related to theparticular plastic resin used for molding. One common method to changethe physical properties of a molded part is to add certain compounds tothe plastic resins, prior to melting and molding (for example, the glassfiller mentioned above). Thus, the physical properties of a plastic partcan generally be controlled by selecting and adding compounds to theplastic resin used for the molding process. Although only onecharacteristic has been discussed (increasing stiffness though theaddition of a glass filler to the plastic resin), other characteristicsmay be changed in a similar manner. By way of example and notlimitation, other filler examples are wood flour, calcium carbonate andaluminum powder. Also, other physical characteristics may becharacterized other than strength. Other physical properties of interestcan be tensile strength, flexural strength, hardness, compressivestrength, impact strength among many others.

Thus, by selecting two materials (such as two different compounds ofplastic resins), and simultaneously injecting both materials into amold, a visually continuous part can be made, with separate regions,each having different physical properties.

FIG. 3 shows another embodiment of an injection molding system 300 inaccordance with the specification. The system 300 includes a mold 310, afirst gate 320, a second gate 322 and a third gate 324. A first material330 is injected into the mold 310 through gate 320 and 324. A secondmaterial 332 is injected into the mold 310 through gate 322. As shown,the first material 330 is positioned in regions near the gates 320 and324 while the second material 332 is positioned near the gate 322. Otherembodiments can have more gates and can use more than two materials.

The position of the gates 320, 322 and 324 with respect to the mold 310as well as injection characteristics such as injection pressure,material temperature and working viscosity can determine how the flowfronts of the material form, propagate, and, ultimately form interfaceregions 340 and 341. Position of the interface regions 340 and 341 inthe mold 310 can determine the final position of the materials 330 and332 in the injection molded part. Thus, by controlling the flow frontsof the injected material, a designer can control the position of thematerials in the final injection molded part. If the first material 330has different physical properties than the second material 332, thendifferent regions (related to the first material 330 and the secondmaterial 332) of the injection molded part can possess the physicalproperties related to the injected material.

FIG. 4 shows an exemplary injection molded part 400 made in accordancewith the specification. Although the mold that produces part 400 is notshown for clarity, exemplary sprues 410, 412 and 414 are shown (spruescan be coupled to gates on the mold and may be disposed between barrelssuch as barrel 220 and gates such as gate 230). Furthermore, the sprues410, 412 and 414 are illustrated as they may be positioned relative toeach other and with respect to the mold. The dashed lines 430 from thesprues 410, 412 and 414 to the part 400 illustrate the relationshipbetween regions on the part 400 and the sprues 410, 412 and 414. Thus,region 420 can be related to material from sprues 410 and 412, while theregion 422 within the dashed line can be attributed to material fromsprue 414.

For example, if the bulk of part 400, requires stiffness, while asection of the part requires relatively more flexibility than the bulk,then the material comprising region 420 can be made relatively stifferthan region 422. In one embodiment, if a plastic resin is used for bothregions 420, 422, a glass filler can be added to the polymer used inregion 420 to add stiffness.

Continuing this example, by using similar materials with differentphysical properties (at least one physical property differs between thesimilar materials), a molded part can be produced with at least tworegions, each region with a different physical property. Furthermore,since the two materials are injected into the mold relativelysimultaneously, the resulting molded part can appear smooth, uniform andcontinuous, even across interface region 440. Through the positioning ofgates with respect to the mold and controlling the flow fronts of thematerials injected into the mold, the position of multiple regionswithin the final molded part can be determined.

Injection molded parts produced as described herein can have severaladvantages. Regions of different physical properties can be positionedto enhance the performance of molded parts. For example, if a corner ofa molded part can be subject to impacts, the designer may want thatcorner to be composed of a relatively ductile material. However, thesame molded part may require a relatively stiff (and perhaps lessductile) material in other regions. Using the methods described above, asingle molded part can be injection molded with a ductile region (suchas region 422 in FIG. 4) and a relatively stiffer region (such as region420 in FIG. 4). The stiffness and/or ductility of the regions can becontrolled by adding effective amounts of filler to plastic resins thatcan be selected for the injection molded part. Also, since the twomaterials are injected substantially simultaneously into the mold, theresulting molded part can have a uniform, smooth appearance with nodelineation between materials.

FIG. 5 is a flowchart of method steps 500 for molding a part withnon-uniform material properties in accordance with the specification.Persons skilled in the art will understand that any system configured toperform the method steps in any order is within the scope of thisdescription.

The method begins in step 502 where a first material is selected. Instep 504, a second material is selected. Often times, the first materialand the second material can both be plastic resins. The first materialcan differ from the second material by the addition of a component meantto change at least one physical property after the material has meltedand cooled (i.e., after molding). For example, a glass filler can beadded to the first material to affect the stiffness of the molded partwhere the first material is disposed.

In step 506, the first and second materials are prepared for injection.In one embodiment, the materials are mixed, placed in hoppers, fed fromhopper and heated. Often, the materials are heated in the barrels suchas barrels 120 and 122 shown in FIG. 1. In Step 508, the first andsecond materials are relatively simultaneously injected into the mold101. The term relatively simultaneously is used to indicate that bothmaterials are injected into the mold at substantially the same time. Instep 510, the flow fronts of the first material and the second materialare controlled. Controlling the flow fronts allows the designer tocontrol the position of the first and second materials with respect tothe molded part. After the heated material cools, in step 512, themolded part is ejected from the mold. Since two different materials withtwo different physical properties are selected, the resulting part canhave two or more regions, each region with a characteristic physicalproperty related to the material selected in steps 502 and 504. Themethod then ends.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of specific embodimentsare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the described embodiments to theprecise forms disclosed. It will be apparent to one of ordinary skill inthe art that many modifications and variations are possible in view ofthe above teachings.

What is claimed is:
 1. A method for molding a uniform part withnon-uniform physical properties, the method comprising: selecting afirst material; selecting a second material, wherein the second materialhas at least one different physical property than the first material;simultaneously injecting the first and second materials into a mold; andcontrolling flow fronts of the first and second materials in the mold.2. The method of claim 1, further comprising: producing a uniform moldedpart with a first region having a first physical property and a secondregion having a second physical property different from the firstphysical property.
 3. The method of claim 1, wherein the first materialis injected through a first gate and the second material is injectedthrough a second gate.
 4. The method of claim 1, wherein the first andsecond materials are plastic resins.
 5. The method of claim 4, whereinthe first material includes a glass filler.
 6. The method of claim 1,wherein the flow fronts determine positions of the physical propertieson the part
 7. The method of claim 1, the controlling flow frontscomprising: controlling positions of the first gate and the second gateon the mold; controlling injection pressure of the first and secondmaterials; and controlling temperatures of the first and secondmaterials.
 8. The method of claim 2, wherein the first region has adifferent stiffness from the second region.
 9. The method of claim 2,wherein the first region has a different hardness from the secondregion.
 10. The method of claim 2, wherein the first region has adifferent flexural strength from the second region.
 11. An apparatus forinjection molding a part with non-uniform material properties, theapparatus comprising: a first hopper configured to hold a firstmaterial; a second hopper configured to hold a second material; a moldincluding a first gate and a second gate; a first barrel, coupled to thefirst hopper and coupled to the first gate, the first barrel disposedbetween the first hopper and the first gate, the first barrel meltingthe first material; a second barrel, coupled to the second hopper andcoupled to the second gate, the second barrel disposed between thesecond hopper and the second gate, the second barrel melting the secondmaterial; and, an injector controller coupled to the first and thesecond barrels, configured to coordinate the injection of the meltedfirst and the second materials simultaneously into the mold.
 12. Theapparatus of claim 11, wherein the first material has a differentphysical property than the second material.
 13. The apparatus of claim11, wherein the first and second materials are plastic resins and thefirst material includes a glass filler.
 14. The apparatus of claim 11,wherein the molded part produced by the mold has a first region and asecond region.
 15. The apparatus of claim 14, wherein the first regionhas at least one physical property different from the second region. 16.An apparatus for injection molding a part with non-uniform materialproperties, the apparatus comprising: means for holding a firstmaterial; means for holding a second material; means for melting thefirst and second materials; means for simultaneously injecting the firstand second melted materials; and means for receiving the injected firstand second materials;
 17. The apparatus of claim 16 wherein thesimultaneously injecting the first and second melted materialscomprises: means for controlling the injection of the first materialthrough a first gate; and means for controlling the injection of thesecond material through a second gate.
 18. The apparatus of claim 16,further comprising: means for controlling a flow front of the firstmelted material; and means for controlling a flow front of the secondmelted material.